Semiconductor memory devices for storing data may be classified into two groups, volatile memory devices and non-volatile memory devices. The volatile memory devices are typically configured to store data by charging or discharging capacitors in memory cells, and widely used as main memories of various apparatuses. The volatile memory devices such as Dynamic Random Access Memory (DRAM) and Static Random Access Memory (SRAM) retain stored data while power is supplied and lose the stored data when power is off. The volatile memory devices are used as a main memory device in various apparatuses, such as computers.
The non-volatile memory devices, such as flash memory devices, may maintain stored data even though power is off, and widely used for storing program codes and/or data in computers, mobile devices, etc.
According to demands for high memory capacity, high operation speed and low power consumption of the memory devices, new memory devices of various types have been developed to realize high integration rate of DRAM, high speed of SRAM and non-volatility of flash memory in a single memory device. For example, Phase Change Random Access Memory (PRAM) that use phase change materials, Resistance Random Access Memory (RRAM) that use materials having variable resistance such as transition-metal oxides, and Magnetic Random Access Memory (MRAM) that use ferromagnetism materials are attracting attention as memory devices of the next generation. Such materials have common characteristics that resistance thereof is variable depending on magnitude and/or direction of applied voltage and/or current, and that the resistance can be maintained (that is, non-volatility) even though the applied voltage and/or current is intercepted and thus refresh operation is not required.
Each memory cell of the resistive memory devices may be formed with one resistive element and one switching element so that data may be stored by controlling voltage and/or current of a bitline and a wordline to change resistance of the resistive element.
The resistive memory cells may be classified into PRAM, RRAM, MRAM, etc. depending on materials forming the resistive element of the memory cell. For example, the resistive element of PRAM may include phase change materials such as Ge—Sb—Te, resistance of which is varied depending on temperature, the resistive element of RRAM may include a top electrode, a bottom electrode and transition-metal oxides between the top and bottom electrodes, and the resistive element of MRAM may include a magnetic top electrode, a magnetic bottom electrode and dielectric materials between the top and bottom electrodes.
The resistive memory device may be classified into a uni-directional resistive memory device, in which resistance of the resistive element changes according to magnitude of voltage and/or current of a bitline and a wordline, and a bi-directional resistive memory device, in which resistance of the resistive element changes according to magnitude and direction of voltage and/or current of a bitline and a wordline.